The present invention relates to a method of and an apparatus for forming a multi-layer film for use in an optical device, etc.
Conventionally, monochromatic photometry or bicolored photometry has been employed for forming a multi-layer film. A known multi-layer film forming apparatus based on monochromatic photometry is described with reference to FIG. 1. In FIG. 1, deposition material 183 is heated for its fusion by an electron beam gun 182 so as to be evaporated. The electron beam gun 182 is provided in a vacuum chamber 181 which is held in a high vacuum state by an exhaust pump 180. Evaporated particles 184 of the deposition material 183 proceed to a substrate 186 mounted on a substrate holder 185 so as to form a thin film on the substrate 186. Meanwhile, an optical film thickness monitoring substrate 188 for monitoring an optical thickness of a thin film is provided above an aperture 187 of the substrate holder 185. The evaporated particles 184 which have passed through the aperture 187 of the substrate holder 185 reach the thickness monitoring substrate 188 so as to form a thin film on the thickness monitoring substrate 188.
A bundle of rays of a specific wavelength emitted from a light source 189 reaches the thin film formed on the thickness monitoring substrate 188 and is reflected by the thin film formed on the thickness monitoring substrate 188 so as to be detected by a detector 190. The amount of reflected light from the thin film formed on the thickness monitoring substrate 188 varies according to the thickness of the thin film. Therefore, when the amount of reflected light from the thin film formed on the thickness monitoring substrate 188 has reached a predetermined value, a shutter 191 is closed so as to terminate deposition of the deposition material 183 on the substrate 186 such that the formation of the thin film of a first layer on the substrate 186 is completed. As a result, the required thickness accuracy of the thin film on the substrate 186 is achieved.
Subsequently, the deposition material 183 is replaced by another deposition material 183 and the thickness monitoring substrate 188 is also replaced by a new substrate. In the same manner as the first layer, the deposition material 183 is heated for its fusion by the electron beam gun 182 so as to be evaporated such that a thin film of a second layer is formed on the substrate 186. By repeating such procedures a plurality of times, a multi-layer film is formed.
However, the above mentioned known method has the following drawbacks. Namely, since the thickness of the thin film formed on the substrate is determined in accordance with the amount of light reflected from or transmitted through the thin film formed on the optical film thickness monitoring substrate, it is extremely difficult to perform a highly accurate control of the thickness of the thin film formed on the substrate to within a 0.5% accuracy.
Meanwhile, the amount of light reflected from or transmitted through the thin film formed on the optical film thickness monitoring substrate varies periodically according to an optical film thickness of the thin film, which is equal to a product of a refractive index of the thin film multiplied by a thickness of the thin film. Thus, by employing this periodicity, the optical film thickness of the thin film can be controlled comparatively easily. However, when the refractive index of the thin film has changed due to minute changes of thin film forming conditions such as degree of vacuum, temperature, etc., the thickness of the thin film varies even if the optical thickness of the thin film is constant, so that it is difficult to perform highly accurate control of the thickness of the thin film.
Since specifications required of recent optical devices are severe, the conventional accuracy in optical film thickness of the thin film is insufficient for the specifications and thus, there is a keen demand for a more accurate method of controlling the thickness of the thin film.
Meanwhile, even if only one layer has an improper thickness for some reason or other in a multi-layer film having several tens of layers, a batch consisting of all of the products employing the multi-layer film become defective, which constitute an obstacle to mass production of the products.